The invention relates to wireless communications systems, and more particularly relates to wireless communications systems for providing remote wireless access to computer networks.
The personal computer has become ubiquitous as a business tool, and the rapid increase in popularity of the laptop or notebook computer has accelerated this phenomenon. The laptop personal computer has now achieved sufficient performance that users frequently rely on their laptops as their sole personal computer.
At the same time, the ever-expanding need for access to larger and larger stores of data has made the availability of networks, including LAN""s, WAN""s, the Internet, the World-Wide Web, and other large databases equally important to the future of computing. However, in contrast to the freedom offered by laptops, access to such large stores of information typically requires a wired connection to a local area network or to the Public Switched Telephone Network (PSTN). Thus, while laptops offer significantly greater freedom in permitting the user to choose where and when to work, the coexisting requirement of a wired connection limits that freedom.
Some products have been offered which attempt to resolve this dilemma. The most common such product is a modem capable of connecting through a conventional cellular telephone. The difficulty with such products is the slow data rate achievable by such products. While the concept is attractive, high speed wireless communications compliant with the v.34 standard, or communications at speeds on the order of 28.8 Kbps or 33.6 Kbps, requires a virtually noise-free communications signal with substantially no data loss, and this performance level has not been achievable in the existing commercially available products.
In addition, there have been specialty devices which integrate cellular or other wireless connections into modems. While these devices offer the advantage of higher levels of integration, they suffer from essentially the same performance limitations as modem/cell phone combinations.
Moreover, such conventional devices do not address an increasingly significant need of the home/small business computer user market. Many such computer users, and especially users of laptops in these environments, prefer to work at various locations within their home or office even though phone connections may only be available in one of those locations. With conventional systems, the user is required to disconnect from the public switched telephone network to be able to operate at their desired location. This limits the freedom of the user in the same manner as described above.
One of the major reasons that existing designs have been unsuccessful in providing adequate performance in the wireless environment has been that wireless operation at high speed requires very good signal-to-noise ratios. In general, the analog devices proposed in the prior art have been unable, at least so far, to maintain an adequate signal to noise ratio across a wireless interface to maintain the overall throughput required for v.34 performance.
As a result, there has been an increasingly apparent need for a system and method which provide the same performance as land lines, but permit the computer user the freedom promised by laptop computers.
The present invention provides a system and method for maintaining a high data rate communications link between a computer system and a remotely located connection to the Public Switched Telephone Network (PSTN). In particular, the system and method of the present invention provide sufficiently reliable v.34-level wireless communications between a personal computer and a remote receiver attached to, for example, the PSTN.
To achieve the high performance required, the analog signals from the modem associated with the computer are converted to digital signals which are then transmitted digitally to a receiver, de-converted back to analog and provided to the conventional PSTN for further propagation to their eventual destination.
While the conversion of the analog modem output to a digital signal is a significant improvement over typical prior art systems, a significant part of the present invention has been the identification of various problems associated with conventional wireless communications techniques when applied to high speed wireless transmission of analog data streams. To assist in maintaining good signal-to-noise ratios and thus in achieving more reliable performance, additional steps may be taken beyond just conversion to a digital data stream. Some form of error correction would prove helpful.
In a real-time digital communications environment, feedback error correction has proved inadequate as having excessive and unpredictable latency. Instead, forward error correction has been found to have significantly better performance for transmission of analog signals both because the latency is fixed (rather than variable) and because the latency is less. Forward error correction is capable of reconstructing bitstreams in which the errors occur on a substantially random basis. However, forward error correction when used alone has been found to be of limited use in a digital wireless environment, because errors in such an environment tend to occur as bursts in which an entire sequence of data bits is lost. This results in the correction algorithm being unable to reconstruct or inaccurate when reconstructing the sequence.
To remedy the difficulty, the system and method of the present invention re-introduce an element of randomness into the errors which occur by interleaving, or re-ordering, units (e.g., bits, words, or code words) of the transmitted data stream. Thus, when burst errors occur, they affect only nonsequential units. The data stream is then de-interleaved and reassembled in its original order at the receiver side.
Although the foregoing elements provide substantially enhanced performance in a wireless environment, additional difficulties such as echo impact the high speed wireless delivery of analog data. Echo and the related issues of timing jitter and frequency offset, while present in prior art systems, take on a different form and impose significantly different concerns in the environment of the present invention. Echo involves leakage between the outgoing data stream and the incoming data stream. While this phenomenon is well known in analog modems where it can be handled through conventional echo cancellation techniques, in the present invention, the echo comes back through the wireless channel and presents much longer delays than conventional techniques can reliably handle. As a result, the present invention permits cancellation of the leakage signal before transmission. The system and method implement special echo cancellation training techniques to train the echo cancellation circuit more rapidly than with conventional analog modems, and also to train the circuit in the presence of various tone noises.
As noted above, timing jitter and frequency offset are other problems which affect performance of a wireless link connecting two analog modems. Frequency offset and timing jitter are both related to the synchronicity between the clocks associated with the A/D and D/A converters in the local and base transceivers. In an exemplary embodiment, the difficulty is overcome by use of a timing recovery technique so that looped time is used for all clocks. By this approach, synchronicity is maintained between the transceivers.
The foregoing summary of the invention may be better understood from the following detailed description and figures.